A maraging steel is a steel containing a large amount of Ni, Mo, Ti, Co, etc. as strengthening elements and is a steel of a type that undergoes age hardening in a martensitic state by heat treatment, and a maraging steel is an ultrahigh-strength steel capable of achieving a very high tensile strength of around 2,000 MPa.
Since a maraging steel has a high tensile strength, a maraging steel is used as a material suitable particularly for a member requiring high strength, such as aerospace or aircraft structural member, a continuously variable transmission component of an automotive engine, a high-pressure vessel, a tool material, a metal mold, etc.
The strengthening mechanism of a maraging steel is attributable to precipitation hardening of an intermetallic compound such as Ni—Ti and Ni—Mo in an aging treatment, and as a representative compositional example thereof, Fe-18Ni-9Co-5Mo-0.4Ti-0.1Al steel has been conventionally known.
However, a maraging steel has a problem that Ti added to steel reacts with N present in the steel to produce a coarse angular TiN inclusion and the TiN inclusion works out to a fracture origin and reduces the fatigue characteristics. Among others, the reduction in fatigue characteristics originated from a coarse TiN inclusion becomes a serious problem, for example, in thin steel sheet having a thickness of 0.5 mm or less, and it is required to solve this problem.
Incidentally, as a related art of the present invention, Patent Document 1 discloses an invention of “Method for Processing and Heat-Treating Maraging Steel”, and a composition of a Zr-containing maraging steel is disclosed in claims thereof.
However, Patent Document 1 does not refer to Zr at all in description as well as in Examples where Zr must have added specifically, and differs from the present invention.
As another related art, Patent Document 2 discloses an invention of “Ultrahigh Tensile Strength High-Toughness Steel” and states in claim 2 that Zr can be added as a selective element. However, the steel described in Patent Document 2 has a low Ni content of 4.1 to 9.5 and in addition, Patent Document 2 describes no Example where Zr is added, and differs from the present invention.
As still another related art, Patent Document 3 discloses an invention of “Ultrahigh Strength Steel”, and a composition containing Zr as one of selective elements is disclosed in claim 1 thereof. However, the steel described in Patent Document 3 has a Co content of 15.0 to 21.0, which is a high content compared with that of the present invention, and furthermore, the invention in Patent Document 3 differs from the present invention in that the reason for adding Zr is to enhance cleanliness by deoxygenation and enhance ductility by denitrogenation and prevention of grain boundary precipitation of Mo and Cr.
As yet still another related art, Patent Document 4 discloses an invention of “Maraging Steel Excellent in Heat Checking Resistance”, and incorporation of Zr is disclosed in claim 1 thereof.
However, the steel described in Patent Document 4 has a low Ni content of 6.0 to 11.0, and the invention in Patent Document 4 differs from the present invention.
[Patent Document 1] JP-A-51-87118
[Patent Document 2] JP-A-53-30916
[Patent Document 3] JP-A-58-25457
[Patent Document 4] JP-A-7-243003